Fix bootstrap

See merge request !196

Userspace fexec

See merge request !195

This is, other than vfork, the last piece of functionality that the
previous clone() offered (CLONE_SIGHAND) which previously was not
implemented.

This fixes file descriptor leaks. Suppose relibc is just about to set
the address space. For this, it needs to write the address space fd to
the selection fd. To avoid having to close them in the kernel, it rather
memorizes what the file descriptors refer to internally, and then do the
actual operation when they are gone, i.e. when closing.